This invention relates to a porous membrane having bulk properties which differ from its surface properties and to a process for making the same. More particularly, this invention relates to a microporous or ultrafiltration membrane formed from a hydrophobic substrate having a hydrophilic surface including the membrane pore surfaces and to a process for forming such a membrane.
In many applications of filtration technology, it is desirable to utilize a membrane filter which is mechanically strong, is thermally stable, is relatively inert chemically and is insoluble in most organic solvents. Often, it is desirable that the membrane have surface properties which are radically different from, and sometimes incompatable with the bulk properties set forth above. Desirable surface properties include wettability, low protein adsorbing tendency, thromborestivity, controlled ion exchange capacity and controlled surface chemical reactivity.
Conventional methodology presently used to achieve the duality of function of bulk properties which differ from the surface properties is to coat a performed membrane having the desired bulk properties with an oligomer or polymer having the desired surface properties. Typical coating materials include surfactants and water soluble polymers such as polyvinylpyrrollidone. This approach to modifying surface properties is undesirable since the coating is only temporary and exposure to any process fluid, particularly when the substrate having the desired bulk properties is a porous membrane, effects removal of the coating from the porous membrane. Membranes treated in this fashion cannot be steam sterilized, cannot be rewet once dried after being wetted with water and exhibit high extractable levels. These properties are unacceptable in many filtration applications, particularly when processing biological fluids which are to be sterilized or subsequently analyzed.
It also has been proposed to utilize graft polymerization techniques to modify the surface characteristics of a polymeric substrate. Typical examples of graft polymerization are shown for example in U.S. Pat. Nos. 3,253,057; 4,151,225; 4,278,777 and 4,311,573. It is difficult to utilize presently available graft polymerization techniques to modify the surface properties of the porous membrane. This is because it is difficult to modify the entire surface of the membrane including the surfaces within the pores while avoiding pore blockage and while retaining membrane porosity. In U.S. Pat. No. 4,340,482, issued July 20, 1982, it has been proposed to modify the surface of porous membranes formed from hydrophobic fluorine-containing polymers by binding a primary amine such as glycine to the hydrophobic substrate. The primary amine renders the polymer surface hydrophilic and can be utilized as a reactant site to link a polymerizable monomer to the porous membrane thereby to obtain a porous membrane having surface properties corresponding to that of the polymerized monomer. Unfortunately, the modified membranes so-produced exhibit properties which are undesirable for use with certain materials. Thus, the membrane so-produced oftentimes is colored that is, a nonwhite color, and gives off colored extractable compositions during use. Furthermore, the membranes have a tendency to adsorb proteins from solution and therefore are unacceptable in some applications such as in clinical diagnostic assays. Accordingly, it would be highly desirable, for example, to provide a composite membrane having both desirable bulk physical strength and chemical resistance while having desired surface properties different from the bulk properties. Furthermore, it would be desirable to provide a membrane which is not colored by virtue of surface modification thereof, which is characterized by very low levels of extractables and which exhibits very low adsorptivity for proteins.